1. Field of the Invention
This invention relates to a method for producing steel castings by continuous casting process.
2. Description of the Prior Art
In continuous steel casting, there arise problems of defects as detected by ultrasonic test, e.g., inclusions occurring in a sub-surface or internal portion of a continuously cast strand (hereinafter referred to as "c.c. strand" for brevity) in its solidifying stage or shrinkage cavities produced in axial center portions of the c.c. strand. In addition, strong segregation occurs in c.c. strands cast at high temperature in continuous casting operations, impairing cold forgeability due to lowered reduction ratio.
Various attempts have thus far been made to eliminate the internal defects of c.c. strands, including the center segregations and shrinkage cavities, through single electromagnetic stirring either within a mold or in a secondary cooling zone, severing tip ends of growing crystals with fluidic movements of molten steel to produce a large quantity of equiaxed crystal nuclei, thereby expanding the equiaxed crystal zone in the center portion of c.c. strands. However, none of them has succeeded in sufficiently reducing the rate of center segregation and irregularities of center segregation in the axial direction of c.c. strands, failing to produce steel castings of satisfactory quality.